OK, you've got 2 listboxes: lstBox1 and lstBox2 (for example)

You'll want to do something like this:

VB Code:
  1. For i = 0 To lstBox1.ListCount - 1
  2.       if i = 0 then          
  3.       lstBox2.AddItem "0.00"
  4.       else
  5.       lstBox2.AddItem lstBox2.List(i-1) + lstBox1.List(i)
  6.       end if
  7. Next

And you'll want to put that in the lstBox1_Change event.

Or something like that... I didn't test that. You get the idea?